Pressure transfer means are applied in industrial pressure measuring technology for transferring pressures. Pressure transfer means include, as a rule, a membrane, or diaphragm, carrier, on which the separating membrane, or diaphragm, is welded at its outer edge. A pressure receiving chamber enclosed between the separating membrane and the membrane bed is filled with a pressure transfer liquid, which serves for transferring to another location, e.g. via a pressure transfer line connected to the pressure receiving chamber, a pressure acting externally on the separating membrane.
Typical applications for such pressure transfer means are pressure measuring transducers. Pressure measuring transducer are applied in almost all branches of industry for measuring pressures. The measured pressure values are used, e.g. for control (open, or closed loop, control) and/or for monitoring of an industrial, manufacturing, and/or treatment, process.
Popular in pressure measuring technology is the application of so called “semiconductor sensors”, e.g. silicon chips containing doped resistance elements sensitive to pressure. Usually, such a pressure sensor includes a pressure sensor chip in the form of a membrane mounted in a pressure measuring chamber on a platform. Pressure sensors are, as a rule, very sensitive and are, therefore, not directly subjected to a medium, whose pressure is to be registered. Instead, a liquid filled, pressure transfer means having an outer, separating membrane is interposed. In operation, the pressure to be measured acts on the separating membrane and is transferred via the pressure transfer means into the pressure measuring chamber. The is true both for the measuring of absolute pressures, as well as also for the measuring of relative pressures and pressure differences.
In order to achieve a pressure transfer characteristic which is as linear and hysteresis free as possible, the separating membrane is preferably formed to be wave shaped.
The membrane bed integrated in the membrane carrier provides overload protection for the separating membrane. An overload refers to an acting on the separating membrane of a pressure, which exceeds the measuring range, for which the pressure measuring transducer is designed, or the pressure range for which the pressure transfer means is designed. In such case, it is especially important, that the membrane bed and the separating membrane are as equal in form as possible, so that the separating membrane, in the case of an overload, rests uniformly against the equally formed membrane bed and experiences a uniform support. In this way, the separating membrane, in the case of overload, is protected from damage and/or permanent deformation.
The forming of the separating membrane is usually done today by embossing.
In such case, for example, a planar, separating membrane blank is embossed by means of a metal punch, an elastic, embossing pad or the hydraulic, or pneumatic, embossing method and then welded onto the membrane carrier. Subsequently, the pressure transfer means is filled with the pressure transfer liquid.
This method offers the advantage, that the separating membranes can be prefabricated, wherein they are, for example, cut out and embossed in one working step. It has the disadvantage, that weld stresses, as well as warping of the separating membrane possibly occurring during the welding of the separating membrane onto the membrane carrier, become permanent. These degrade the pressure transfer characteristics of the pressure transfer means. Especially, they act disadvantageously on the linearity and can cause hysteresis.
Moreover, in the case of this method, very high requirements are placed on the manufacturing tolerances for the individual components, in order to achieve, that the separating membrane and its membrane bed are as equal in form as possible. Each still small deviation in the forming can, in the case of overload, lead to a permanent deformation or even to damage of the separating membrane and acts therewith disadvantageously on the accuracy and the reproducibility of the pressure transfer. This effects then, directly, the accuracy of measurement of a pressure measuring transducer equipped with the pressure transfer means.
Alternatively to the aforementioned method, the separating membrane blank can first be welded onto the membrane carrier and then embossed by pressing the blank against the membrane bed. This embossing can happen, for example, hydraulically, pneumatically or by means of an embossing pad. Then, the pressure transfer means is filled with the pressure transfer liquid.
This offers the advantage, that manufacturing tolerances in the forming of the membrane bed are assumed when the separating membrane is embossed by pressing against the membrane bed, and that weld stresses as well as warping of the separating membrane can be, at least partially, cancelled by the embossing procedure. Moreover, the separating membrane experiences, during the embossing against the membrane bed, mechanical loadings similar to those experienced in the case of a later, possibly arising overload.
In the case of the aforementioned embossing method, the forming of the separating membrane depends on the physical character of the medium enclosed, before and during the embossing, between the separating membrane and the membrane bed. Both moisture enclosed in the intermediate space, as well as also dust and/or enclosed particles affect the forming. This has the result, that individual pressure transfer means manufactured in the same way can have very different pressure transfer characteristics. For the use of these pressure transfer means for measuring of pressures, this means, as a rule, that complex calibration, or compensation, methods must be performed for each individual pressure measuring transducer.